The form -(으)ㄹ 텐데 is a pretty nifty form that lets you express what you expect or expected the future to bring. You then further expand on that either by saying the opposite happened or you simply give some additional background about your expectation.
The form -(으)ㄹ 텐데 is made up out of two parts. The first part is -(으)ㄹ 테이다. It expresses a particular future or an intention to do something. The second part is the form -(으)ㄴ/는데, which you use to give background information about certain things. Because of the joining of the two, the form -(으)ㄹ 텐데 expresses the future you suppose or assumed.
When using the form -(으)ㄹ 텐데 both clauses are very important. The first clause indicates the future you expect or the future you expected. In the second clause, you either indicate that what happened is contrary to what you expected. Or, you give some additional background information about what you expect to happen.
Make sure to not mix up the form -(으)ㄹ 텐데 with the form -(으)ㄴ/는데. The first you use to express your supposition or assumption about a current or future situation. The latter you use to either give background information about the discussed topic or give information about what follows in the next clause.
Also do not confuse it with its brother, -(으)ㄹ 테니까. While they appear similar there is a difference in meaning. -(으)ㄹ 테니까 gives the reason in the first clause as to why you give a particular piece of advice or suggestion.
가: 눈이 많이 오네요.
나: 눈이 많이 오면 미끄러울 텐데 밖에 나가면 조심하세요
A: It is snowing a lot.
B: If it is snowing a lot, it ought to be slippery, so when going outside, please be careful.
How To Use
The conjugation of any form, including the form -(으)ㄹ 텐데, should no longer cause you any issues. As you have probably guessed, the present tense form -(으)ㄹ 텐데 follows the basic ‘batching/no batchim’ rule. It also has the rule that a ㄹ batchim is treated as if it were a no batchim. The past tense of this form is -았/었을 텐데 and thus follows the ‘last vowel’ rule.
The form -(으)ㄹ 텐데 can also be used at the end of a sentence in the form of -(으)ㄹ 텐데요. When you use it at the end of a sentence, it can also express your supposition or assumption about what is being discussed.